THIS INVENTION relates to the production of liquid hydrocarbon products. It relates in particular to a process for producing liquid hydrocarbon products.
According to a first aspect of the invention, there is provided a process for producing liquid hydrocarbon products, which process includes
converting, in a synthesis gas production stage, a natural gas feedstock comprising mainly CH4 to synthesis gas, comprising CO, H2, CO2 and CH4;
feeding the synthesis gas, as a feedstock, to a hydrocarbon synthesis stage;
in the hydrocarbon synthesis stage, reacting the synthesis gas at elevated temperature and pressure, and in the presence of a Fischer-Tropsch catalyst, to produce a range of hydrocarbon products of differing carbon chain lengths and separating an overheads vapour phase comprising gaseous hydrocarbon products, unreacted synthesis gas, water, and any soluble organic compounds which form in the hydrocarbon synthesis stage, from a liquid phase comprising heavier liquid hydrocarbon products;
withdrawing the liquid phase from the hydrocarbon synthesis stage;
withdrawing the overheads vapour phase from the hydrocarbon synthesis stage and feeding it to a product condensation stage, where condensation of at least some components of the overheads vapour phase takes place;
withdrawing from the product condensation stage a vapour phase comprising gaseous hydrocarbon products, an aqueous phase comprising water and said any soluble organic compounds which form in the hydrocarbon synthesis stage, and a condensed product phase comprising condensed hydrocarbon products;
feeding the vapour phase to a vapour phase work-up stage;
in the vapour phase work-up stage, recovering from the vapour phase a gas component comprising increased concentrations of CO and H2, relative to the vapour phase feed to the vapour phase work-up stage; and
recycling the CO and H2-containing gas component to the hydrocarbon synthesis stage, as a feedstock component.
The conversion of the natural gas to synthesis gas in the synthesis gas production stage may be effected by any suitable reaction mechanism involving reacting hydrocarbonaceous material, which is primarily CH4, in the natural gas with steam and/or oxygen. Typically, the conversion may be effected by means of steam reforming, which does not require the use of oxygen; autothermal reforming, in which the hydrocarbonaceous material reacts with oxygen in a first reaction section, whereafter an endothermic steam reforming reaction takes place adiabatically in a second reaction section; ceramic oxygen transfer membrane reforming, in which oxygen required for the reforming reaction is transported through an oxygen permeable membrane into a reaction zone; plasma reforming in which the reforming reaction is driven by an electrically generated plasma; non-catalytic partial oxidation; or catalytic partial oxidation. If desired, two or more of these conversion mechanisms or technologies may be combined, eg to optimize thermal efficiency, or to obtain an optimised or beneficial synthesis gas composition.
The present invention is characterized thereby that it is not necessary to remove CO2 from the synthesis gas before using it as feedstock to the hydrocarbon synthesis stage.
The hydrocarbon synthesis stage may include a suitable reactor such as a tubular fixed bed reactor, a slurry bed reactor or an ebullating bed reactor. The pressure in the reactor may be between 1 and 100 bar, while the temperature may be between 200xc2x0 C. and 380xc2x0 C. The reactor will thus contain the Fischer-Tropsch catalyst, which will be in particulate form. The catalyst may contain, as its active catalyst component, Co, Fe, Ni, Ru, Re and/or Rh. The catalyst may be promoted with one or more promoters selected from an alkali metal, V, Cr, Pt, Pd. La, Re, Rh, Ru, Th, Mn, Cu, Mg, Zn and Zr. The catalyst may typically be a supported catalyst, in which the active catalyst component, eg Co, is supported on a suitable support. The support may be Al2O3, TiO2, SiO2 or a combination of these.
In the hydrocarbon synthesis stage, the synthesis gas is thus catalytically reacted by means of so-called Fischer-Tropsch synthesis. Thus, for example, low temperature Fischer-Tropsch synthesis with a co-based catalyst may be used. The reaction temperature will then typically be in the range of 200xc2x0 C. to 260xc2x0 C. A Co-based catalyst does not exhibit any significant water gas shift activity. Thus, in low temperature Fischer-Tropsch synthesis using a Co-based catalyst, the main reactants are H2 and CO, with CO2 in the synthesis gas behaving as an inert gas in the hydrocarbon synthesis stage.
The condensed product phase that is withdrawn from the product condensation stage typically comprises hydrocarbon products having 3 or more carbon atoms.
In the vapour phase work-up stage, the vapour phase may be separated into the gas component comprising the increased concentrations of CO and H2 (hereinafter also referred to as the xe2x80x98first gas componentxe2x80x99), a second gas component enriched in CH4, and, optionally, a third gas component comprising mainly CO2.
In one embodiment of the invention, the third gas component may be present. The vapour phase work-up stage may then include a CO2 removal step in which the third gas component is removed from the vapour phase, and a subsequent cryogenic separation step to which the residual vapour phase is subjected and in which the first gas component is cryogenically separated from the second gas component.
In another embodiment of the invention, the vapour phase work-up stage may include a heavy ends recovery step in which hydrocarbon products having 3 or more carbon atoms, and which are present in the vapour phase, are removed from the vapour phase; the residual vapour phase may then pass to a subsequent pressure swing adsorption step where it is separated into the first and second gas components, and, optionally, the third gas component. The third gas component, when present, will comprise mainly CO2 and some light hydrocarbon products.
When present, the third gas component may be used as a fuel gas, for example, in the synthesis gas production stage and/or for superheating process steam and other uses.
The second gas component may be used to satisfy any remaining fuel gas demand; optionally, as a feedstock to a hydrogen production stage in which hydrogen is produced from CH4; and, optionally, in the synthesis gas production stage.
When hydrogen is produced from the second gas component, it may be added to the synthesis gas feedstock to the hydrocarbon synthesis stage, thereby to increase the synthesis gas hydrogen content. Instead, or additionally, hydrogen thus obtained may be used to upgrade the liquid hydrocarbon products produced in the hydrocarbon synthesis stage, as described in more detail hereunder.
An advantage of using the second gas component for hydrogen production, is that no treatment thereof is required for the removal of sulphur therefrom, since the second gas component is sulphur free.
The remainder of the second gas component, ie any residual second gas component not required for fuel gas or for hydrogen production, may be recycled as a feedstock component to the synthesis gas production stage. However, it will then be necessary to compress the gas to the same pressure as the natural gas feedstock to the synthesis gas preparation stage. Since the second gas component may still contain some CO2, CO and H2, it is less desirable for use as a feedstock component in the synthesis gas production stage.
The process may further include, in a liquid product upgrading stage, upgrading the liquid hydrocarbon products in the liquid phase withdrawn from the hydrocarbon synthesis stage as well as the hydrocarbon products in the condensed product phase from the product condensation stage. This upgrading may be effected by hydroprocessing the hydrocarbon products using hydrogen obtained from the second gas component as hereinbefore described, ie hydrogen produced in the hydrogen production stage.
According to a second aspect of the invention, there is provided a process for producing liquid hydrocarbon products, which process includes
converting; in a synthesis gas production stage, a natural gas feedstock comprising mainly CH4 to synthesis gas comprising CO, H2, CO2 and CH4;
without removal of CO2, feeding the synthesis gas, as a feedstock, to a hydrocarbon synthesis stage;
in the hydrocarbon synthesis stage, reacting the synthesis gas at elevated temperature and pressure, and in the presence of a Co-based Fischer-Tropsch catalyst, to produce a range of hydrocarbon products of differing carbon chain lengths, and separating a vapour phase comprising gaseous hydrocarbon products and unreacted synthesis gas from a liquid phase comprising heavier liquid hydrocarbon products;
withdrawing the liquid phase from the hydrocarbon synthesis stage;
withdrawing the vapour phase from the hydrocarbon synthesis stage and feeding it to a heavy ends recovery stage;
in the heavy ends recovery stage, separating hydrocarbon products having 3 or more carbon atoms, from the vapour phase; and
recycling at least a portion of the vapour phase from the heavy ends recovery stage to the synthesis gas production stage, as a feedstock component.
In the second aspect of the invention, the synthesis gas production stage and the hydrocarbon synthesis stage may be as hereinbefore described.
The invention will now be described by way of example with reference to the accompanying drawings.